Switching regulator

ABSTRACT

A switching regulator includes a switching element, a rectifier element, an output capacitor having one electrode connected to an output terminal, a control circuit which supplies a pulse width modulation signal in accordance with a voltage of the output terminal to a control terminal of the switching element, a load determination circuit which outputs a determination signal in accordance with a load, based on a voltage of the control terminal of the switching element, and a variable inductance circuit including a plurality of coils and having an inductance value which is switchable based on the determination signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 62/812,531 filed on Mar. 1, 2019, and entitled“Switching Regulator”, which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a switching regulator using a choppermethod.

Description of Related Art

Switching regulators are widely used as a conversion circuit of a directcurrent voltage. The switching regulators include those for stepping upan input voltage, those for stepping down the input voltage, those forinverting the input voltage, and the like. In the following, theswitching regulator using a chopper method will be considered.

FIG. 8 is a circuit diagram of a conventional step-up switchingregulator using the chopper method. A switching regulator 9 shown inFIG. 8 includes a coil 91, a field effect transistor (hereinafterreferred to as FET) 92, a diode 93, and a capacitor 94. A pulse widthmodulation signal is supplied from a control circuit (not shown) to agate terminal of the FET 92. In an ON period of the FET 92, energy isaccumulated in the coil 91. In an OFF period of the FET 92, the energyaccumulated in the coil 91 is released to a side of the diode 93. Bycontrolling the FET 92 to an ON state and an OFF state in this manner,an input voltage Vin can be stepped up.

Related to the invention of the present application, Japanese Laid-OpenPatent Publication No. 2016-131464 discloses a DC/DC convertor in whicha conversion section is connected to a primary winding of a transformerand a rectifier section and a smoothing coil are connected to asecondary winding of the transformer. In the case of a large current,the DC/DC convertor increases an inductance value of the smoothing coiland operates in a continuous mode, and in the case of a small current,the DC/DC convertor decreases the inductance value of the smoothing coiland operates in a discontinuous mode.

In the switching regulator 9 shown in FIG. 8, the ON period of the FET92 is controlled to be long when a load is heavy, and is controlled tobe short when the load is light. However, when the load is lighter thana predetermined value, the switching regulator 9 operates in thediscontinuous mode (see FIG. 9). In the discontinuous mode, anefficiency of the switching regulator 9 drops.

As a method for making the switching regulator 9 operate in thecontinuous mode even when the load is light, a method of increasing aninductance value of the coil 91 can be considered. However, when theinductance value of the coil 91 is increased, an amount of current whichcan flow through the coil 91 decreases. Furthermore, in order to make asame amount of current flow through the coil 91, it is necessary to usea large-sized coil or a high-cost coil. In this manner, the method ofincreasing the inductance value of the ceil 91 is difficult to apply infact.

SUMMARY OF THE INVENTION

Therefore, providing a switching regulator which operates efficientlyeven when a load is light to some extent without greatly increasingcircuit size and cost is taken as a problem.

(1) A switching regulator according to some embodiments of the presentinvention includes: a switching element; a rectifier element; an outputcapacitor having one electrode connected to an output terminal; acontrol circuit configured to supply a pulse width modulation signal inaccordance with a voltage of the output terminal to a control terminalof the switching element; a load determination circuit configured tooutput a determination signal in accordance with a load, based on avoltage of the control terminal of the switching element; and a variableinductance circuit including a plurality of coils and having aninductance value which is switchable based on the determination signal.

In the above-described switching regulator, the inductance value of thevariable inductance circuit is switched based on the determinationsignal in accordance with the load. Thus, the switching regulatoroperates in a continuous mode even when the load is light to someextent. Therefore, a switching regulator which operates efficiently evenwhen the load is light to some extent can be provided without greatlyincreasing circuit size and cost.

(2) The switching regulator according to some embodiments of the presentinvention has the configuration of above (1), the variable inductancecircuit includes: a first coil; a second coil; and a second switchingelement connected in parallel with the second coil and configured toturn on when the determination signal indicates a heavy load and to turnoff when the determination signal indicates a light load, and the firstcoil is connected in series with a parallel connection circuit of thesecond coil and the second switching element.

(3) The switching regulator according to some embodiments of the presentinvention has the configuration of above (1), the variable inductancecircuit includes: a first coil; a second coil; and a second switchingelement connected in series with the second coil and configured to turnon when the determination signal indicates a heavy load and to turn offwhen the determination signal indicates a light load, and the first coilis connected in parallel with a series connection circuit of the secondcoil and the second switching element.

(4) The switching regulator according to some embodiments of the presentinvention has the configuration of above (1), and the load determinationcircuit includes an integration circuit configured by a resistor and acapacitor and configured to output an average voltage of the voltage ofthe control terminal of the switching element.

(5) The switching regulator according to some embodiments of the presentinvention has the configuration of above (4), and the load determinationcircuit further includes an inverter configured to output thedetermination signal based on an output signal of the integrationcircuit.

(6) The switching regulator according to some embodiments of the presentinvention has the configuration of above (4), and the load determinationcircuit further includes a buffer configured to output the determinationsignal based on an output signal of the integration circuit.

(7) The switching regulator according to some embodiments of the presentinvention has the configuration of above (4), and the load determinationcircuit further includes: a resistor divider circuit configured togenerate a comparison target voltage based on a reference voltage; and acomparison circuit configured to compare an output voltage of theintegration circuit with the comparison target voltage to output thedetermination signal.

(8) The switching regulator according to some embodiments of the presentinvention has the configuration of above (1), a first terminal of thevariable inductance circuit is connected to an input terminal, a secondterminal of the variable inductance circuit, a first conduction terminalof the switching element, and a first terminal of the rectifier elementare connected to a first node, a second terminal of the rectifierelement is connected to the output terminal, and a second conductionterminal of the switching element and another electrode of the outputcapacitor are grounded.

(9) The switching regulator according to some embodiments of the presentinvention has the configuration of above (1), a first conductionterminal of the switching element is connected to an input terminal, afirst terminal of the rectifier element and another electrode of theoutput capacitor are grounded, a second conduction terminal of theswitching element, a second terminal of the rectifier element, and afirst terminal of the variable inductance circuit are connected to afirst node, and a second terminal of the variable inductance circuit isconnected to the output terminal.

(10) The switching regulator according to some embodiments of thepresent invention has the configuration of above (1), a first conductionterminal of the switching element is connected to an input terminal, afirst terminal of the rectifier element is connected to the outputterminal, a second conduction terminal of the switching element, asecond terminal of the rectifier element, and a first terminal of thevariable inductance circuit are connected to a first node, and a secondterminal of the variable inductance circuit and another electrode of theoutput capacitor are grounded.

(11) The switching regulator according to some embodiments of thepresent invention has the configuration of above (1), and the rectifierelement is a diode.

These and other objects, features, modes and effects of the presentinvention will be more apparent from the following detailed descriptionwith reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a switching regulator according to afirst embodiment.

FIG. 2 is a signal waveform diagram of the switching regulator shown inFIG. 1 in the case of a heavy load.

FIG. 3 is a signal waveform diagram of the switching regulator shown inFIG. 1 in the case of a light load.

FIG. 4 is a circuit diagram of a switching regulator according to asecond embodiment.

FIG. 5 is a circuit diagram of a switching regulator according to athird embodiment.

FIG. 6 is a circuit diagram of a switching regulator according to afourth embodiment.

FIG. 7 is a circuit diagram of a switching regulator according to afifth embodiment.

FIG. 8 is a circuit diagram of a conventional switching regulator.

FIG. 9 is a signal waveform diagram of the switching regulator shown inFIG. 8 in the case of the light load.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

FIG. 1 is a circuit diagram of a switching regulator according to afirst embodiment. A switching regulator 1 shown in FIG. 1 includes avariable inductance circuit 10 (including two coils 11, 16), an FET 12,a diode 13, a capacitor 14, a control circuit 15, and a loaddetermination circuit 20. The FET 12 is an N-channel type FET andfunctions as a switching element. The diode 13 functions as a rectifierelement, and the capacitor 14 functions as an output capacitor. Thecontrol circuit 15 is typically included in a switching regulator IC.The switching regulator 1 is obtained based on a general step-upswitching regulator using a chopper method by replacing the coil 11 withthe variable inductance circuit 10 and adding the load determinationcircuit 20.

As shown in FIG. 1, the variable inductance circuit 10 includes thecoils 11, 16 and an FET 17. The coil 11 functions as a first coil, andthe coil 16 functions as a second coil. The FET 17 is a P-channel typeFET and functions as a second switching element. One end (left-sideterminal) of the coil 11 is connected to an input terminal of theswitching regulator 1. Another end of the coil 11 is connected to oneend (left-side terminal) of the coil 16. Another end of the coil 16 isconnected to a drain terminal of the FET 12 and an anode terminal of thediode 13. A drain terminal and a source terminal of the FET 17 areconnected to the two ends of the coil 16, respectively.

In this manner, the FET 17 is connected in parallel with the coil 16.The coil 11 is connected in series with a parallel connection circuit ofthe coil 16 and the FET 17. A first terminal (left-side terminal) of thevariable inductance circuit 10 is connected to the input terminal of theswitching regulator 1. A second terminal of the variable inductancecircuit 10 is connected to the drain terminal of the FET 12 and theanode terminal of the diode 13.

A cathode terminal of the diode 13 is connected to one electrode(upper-side electrode) of the capacitor 14 and an output terminal of theswitching regulator 1. A source terminal of the FET 12 and anotherelectrode of the capacitor 14 are grounded.

An input terminal of the control circuit 15 is connected to the outputterminal the switching regulator 1, and an output terminal of thecontrol circuit 15 is connected to a gate terminal of the FET 12. Thecontrol circuit 15 supplies, to the gate terminal of the FET 12, a pulsewidth modulation signal S1 in accordance with a voltage Vout of theoutput terminal of the switching regulator 1. The voltage Vout changesin accordance with a load (not shown) connected to the output terminalof the switching regulator 1. Therefore, it can be said that the controlcircuit 15 supplies the pulse width modulation signal S1 in accordancewith the load to the gate terminal of the FET 12. As the load is larger,the control circuit 15 controls a high-level period of the pulse widthmodulation signal S1 to be longer.

The load determination circuit 20 includes a resistor 21, a capacitor22, and an inverter 23. One end (right-side terminal) of the resistor 21is connected to the gate terminal of the FET 12. Another end of theresistor 21 is connected to one electrode (upper-side electrode) of thecapacitor 22 and an input terminal of the inverter 23. Another electrodeof the capacitor 22 is grounded. The resistor 21 and the capacitor 22function as an integration circuit which outputs an average voltage of agate voltage of the FET 12. An output signal of the integration circuitis input to the input terminal of the inverter 23. The inverter 23 is aCMOS inverter, for example, outputs a high-level voltage VH when aninput voltage is lower than a threshold voltage, and outputs a low-levelvoltage VL when the input voltage is higher than the threshold voltage.An output terminal of the inverter 23 is connected to a gate terminal ofthe FET 17.

In the following, a case where the load is larger than a predeterminedvalue is referred to as “heavy load”, and a case where the load issmaller than the predetermined value is referred to as “light load”. Theabove-described predetermined value is determined by a resistance valueof the resistor 21, a capacitance value of the capacitor 22, thethreshold voltage of the inverter 23, and the like. Furthermore, aninductance value of the variable inductance circuit 10 is denoted by H1.In the following, as an example, it is assumed that the high-levelvoltage VH is 3.3 V, the low-level voltage VL is 0 V (ground voltage),an inductance value of the coil 11 is 10 μH, an inductance value of thecoil 16 is 100 μH, capacitance values of the capacitors 14, 22 are 10μF, and the resistance value of the resistor 21 is 10 kΩ.

FIG. 2 is a signal waveform diagram of the switching regulator 1 in thecase of the heavy load. FIG. 3 is a signal waveform diagram of theswitching regulator 1 in the case of the light load. In FIG. 1, a nodeconnected to the gate terminal of the FET 12 is referred to as A, a nodeconnected to the input terminal of the inverter 23 is referred to as B,a node connected to the output terminal of the inverter 23 is referredto as C, and a node connected to the input terminal of the switchingregulator 1 is referred to as D. FIGS. 2 and 3 describe a state of theFET 12, voltages of the nodes A to C, and a current flowing through thenode D.

In the case of the heavy load (FIG. 2), in order to increase a currentflowing through the coil 11, the control circuit 15 controls ahigh-level period of the gate voltage of the FET 12 (voltage of the nodeA) to be long. Thus, an output voltage of the integration circuit(voltage of the node B) comes close to the high-level voltage VH, and anoutput signal S2 of the inverter 23 (voltage of the node C) becomeslow-level. Since the FET 17 turns on at this time, the current flowingthrough the coil 11 flows through the FET 17, but does not flow throughthe coil 16. Therefore, the inductance value H1 in the case of the heavyload is equal to the inductance value of the coil 11 (10 μH). Thecurrent flowing through the node D changes rapidly in accordance withthe inductance value H1 which is relatively small. At this time, theswitching regulator 1 operates in a continuous mode.

In the case of the light load (FIG. 3), in order to decrease the currentflowing through the coil 11, the control circuit 15 controls thehigh-level period of the gate voltage of the FET 12 to be short. Thus,the output voltage of the integration circuit comes close to thelow-level voltage VL, and the output signal S2 of the inverter 23becomes high-level. Since the FET 17 turns off at this time, the currentflowing through the coil 11 flows through the coil 16, but does not flowthrough the FET 17. Therefore, the inductance value H1 in the case ofthe light load is equal to a sum of the inductance value of the coil 11and the inductance value of the coil 16 (110 μH). The current flowingthrough the node D changes slowly in accordance with the inductancevalue H1 which is relatively large. Also in the case shown in FIG. 3,the switching regulator 1 operates in the continuous mode.

In this manner, in the case of the heavy load, the load determinationcircuit 20 outputs a low-level determination signal S2, and the FET 17turns on based on the determination signal S2. At this time, theinductance value H1 becomes a relatively small value, and the switchingregulator 1 operates in the continuous mode. In the case of the lightlead, the load determination circuit 20 outputs a high-leveldetermination signal S2, and the FET 17 turns off based on thedetermination signal S2. At this time, the inductance value H1 becomes arelatively large value. Therefore, the switching regulator 1 operates inthe continuous mode even when the load is light to some extent.

In the following, a circuit obtained by removing the coil 16, the FET17, and the load determination circuit 20 from the switching regulator 1is considered as a switching regulator according to a first comparativeexample. In the switching regulator according to the first comparativeexample, it is assumed that an optimum value of the inductance value ofthe coil 11 in the case of the heavy load is 10 μH. The switchingregulator according to the first comparative example operates in thecontinuous mode in the case of the heavy load, but operates in adiscontinuous mode in the case of the light load. Thus, in the switchingregulator according to the first comparative example, an efficiencydrops in the case of the light load.

Next, a circuit obtained by removing the FET 17 and the loaddetermination circuit 20 from the switching regulator 1 is considered asa switching regulator according to a second comparative example. In theswitching regulator according to the second comparative example, thecoil 11 of 10 μH and the coil 16 of 100 μH are connected in series. Theswitching regulator according to the second comparative example operatesin the continuous mode even when the load is light to some extent.However, the switching regulator according to the second comparativeexample has a problem that size and cost of the coil 16 are increased,as described below.

For example, it is assumed that the switching regulator performsthree-fold step-up, an output current is 100 mA, and a conversionefficiency is 80%. In this case, an input current is 300 mA, and anaverage current flowing through the coils 11, 16 is 375 mA. Consideringthat the current flowing through the coils 11, 16 is a triangle wave,that it is necessary to instantly correspond to an abrupt change of theload, and that it is necessary to provide a margin, the coils 11, 16need to have a current capacitance which is about five times of theaverage current (current capacitance of about 2 A).

A coil having an inductance value of 10 μH and a current capacitance of2 A is sold in a market. The size of the coil is about 5 mm square.However, if the inductance value is to be changed to 100 μH withoutchanging the size of the coil, only a coil having a current capacitanceof 0.74 A is sold in the market. Furthermore, even if the size of thecoil is permitted to be less than about 13 mm square, only a coil havingan inductance value of 100 μH and a current capacitance of 1.4 A is soldin the market. In this manner, the switching regulator according to thesecond comparative example has a problem that the size and the cost ofthe coil 16 are increased.

In the switching regulator 1 according to the present embodiment, it isonly in the case of the light load when the current flows through thecoil 16. Thus, when the load determination circuit 20 which makes theFET 17 turn off when the load is ⅓ of that in the case of the heavy loadis used, only a current of at most a little less than 700 mA flowsthrough the coil 16. Therefore, by using the coil having the inductancevalue of 10 μH, the current capacitance of 2 A, and the size of 5 mmsquare, and the coil having the inductance value of 100 μH, the currentcapacitance of 0.74 A, and the size of 5 mm square which are describedabove, the switching regulator 1 which operates efficiently even whenthe load is light to some extent can be configured without greatlyincreasing the size and the cost.

As described above, the switching regulator 1 according the presentembodiment includes a switching element (FET 12), a rectifier element(diode 13), an output capacitor (capacitor 14) having one electrodeconnected to the output terminal, the control circuit 15 which suppliesthe pulse width modulation signal S1 in accordance with the voltage Voutof the output terminal to a control terminal of the switching element(gate terminal of the FET 12), the load determination circuit 20 whichoutputs the determination signal S2 in accordance with the load, basedon a voltage of the control terminal of the switching element, and thevariable inductance circuit 10 including a plurality of coils 11, 16 andhaving the inductance value which is switchable based on thedetermination signal S2.

In the switching regulator 1 according to the present embodiment, theinductance value H1 of the variable inductance circuit 10 is switchedbased on the determination signal S2 in accordance with the load. Thus,the switching regulator 1 operates in the continuous mode even when theload is light to some extent. Therefore, the switching regulator whichoperates efficiently even when the load is light to some extent can beprovided without greatly increasing the circuit size and the cost.

The variable inductance circuit 10 includes a first coil (coil 11), asecond coil (coil 16), a second switching element (FET 17) which isconnected in parallel with the second coil, turns on when thedetermination signal S2 indicates the heavy load, and turns off when thedetermination signal S2 indicates the light load, and the first coil isconnected in series with a parallel connection circuit of the secondcoil and the second switching element. Therefore, the variableinductance circuit 10 having the inductance value which is switchabiebased on the determination signal S2 can be configured easily.

The load determination circuit 20 includes the integration circuit whichis configured by the resistor 21 and the capacitor 22 and outputs anaverage voltage of the voltage of the control terminal of the switchingelement, and the inverter 23 which outputs the determination signal S2based on the output signal of the integration circuit. Therefore, theload determination circuit 20 which outputs the determination signal S2in accordance with the load can be configured easily by using theintegration circuit and the inverter 23.

A first terminal of the variable inductance circuit 10 is connected tothe input terminal, a second terminal of the variable inductance circuit10, a first conduction terminal of the switching element (drain terminalof the FET 12), and a first terminal of the rectifier element (anodeterminal of the diode 13) are connected to a first node (N1 shown inFIG. 1), a second terminal of the rectifier element (cathode terminal ofthe diode 13) is connected to the output terminal, a second conductionterminal of the switching element (source terminal of the FET 12) andanother electrode of the output capacitor are grounded. Therefore, astep-up switching regulator which operates efficiently even when theload is light to some extent can be provided. Furthermore, the rectifierelement is the diode 13. Therefore, the switching regulator whichoperates efficiently even when the load is light to some extent can beprovided using the diode 13 as the rectifier element.

Second Embodiment

FIG. 4 is a circuit diagram of a switching regulator according to asecond embodiment. A switching regulator 2 shown in FIG. 4 includes thevariable inductance circuit 10 (including the two coils 11, 16), an FET18, the diode 13, the capacitor 14, a control circuit 19, and a loaddetermination circuit 30. The FET 18 is a P-channel type FET andfunctions as a switching element. The switching regulator 2 is obtainedbased on a general step-down switching regulator using the choppermethod by replacing the coil 11 with the variable inductance circuit 10and adding the load determination circuit 30. In the followingembodiments, same elements as those described in any precedingembodiment are provided with the same reference numbers and descriptionthereof is omitted.

As shown in FIG. 4, the configuration of the variable inductance circuit10 is same as that of the first embodiment A source terminal (left-sideterminal) of the FET 18 is connected to an input terminal of theswitching regulator 2. A drain terminal of the FET 18 is connected tothe first terminal (left-side terminal) of the variable inductancecircuit 10 and the cathode terminal of the diode 13. The second terminalof the variable inductance circuit 10 is connected to one electrode(upper-side electrode) of the capacitor 14 and an output terminal of theswitching regulator 2. The anode terminal of the diode 13 and anotherelectrode of the capacitor 14 are grounded.

An input terminal of the control circuit 19 is connected to the outputterminal of the switching regulator 2, and an output terminal of thecontrol circuit 19 is connected to a gate terminal of the FET 18. Thecontrol circuit 19 supplies a pulse width modulation signal S3 inaccordance with a voltage Vout of the output terminal of the switchingregulator 2 (pulse width modulation signal in accordance with the load)to the gate terminal of the FET 18. As the load is larger, the controlcircuit 19 controls a low-level period of the pulse width modulationsignal S3 to be longer.

The load determination circuit 30 includes the resistor 21, thecapacitor 22, and a buffer 31. One end (lower-side terminal) of theresistor 21 is connected to the gate terminal of the FET 18. Another endof the resistor 21 is connected to one electrode (upper-side electrode)of the capacitor 22 and an input terminal of the buffer 31. Anotherelectrode of the capacitor 22 is grounded. The integration circuitconfigured by the resistor 21 and the capacitor 22 outputs an averagevoltage of a gate voltage of the FET 18. The output signal of theintegration circuit is input to an input terminal of the buffer 31. Thebuffer 31 is a CMOS buffer, for example, outputs the low-level voltageVL when an input voltage is lower than a threshold voltage, and outputsthe high-level voltage VH when the input voltage is higher than thethreshold voltage. An output terminal of the buffer 31 is connected tothe gate terminal of the FET 17.

In the case of the heavy load, in order to increase the current flowingthrough the coil 11, the control circuit 19 controls a low-level periodof the gate voltage of the FET 18 to be long. Thus, the output voltageof the integration circuit comes close to the low-level voltage VL, andan output signal S4 of the buffer 31 becomes low-level. Since the FET 17turns on at this time, the current flowing through the coil 11 flowsthrough the FET 17, but does not flow through the coil 16. Therefore,the inductance value H1 in the case of the heavy load is equal to theinductance value of the coil 11.

In the case of the light load, in order to decrease the current flowingthrough the coil 11, the control circuit 19 controls the low-levelperiod of the gate voltage of the FET 18 to be short. Thus, the outputvoltage of the integration circuit comes close to the high-level voltageVH, and the output signal S4 of the buffer 31 becomes high-level. Sincethe FET 17 turns off at this time, the current flowing through the coil11 flows through the coil 16, but does not flow through the FET 17.Therefore, the inductance value H1 in the case of the light load isequal to the sum of the inductance value of the coil 11 and theinductance value of the coil 16.

In this manner, in the case of the heavy load, the load determinationcircuit 30 outputs a low-level determination signal S4, and the FET 17turns on based on the determination signal S4. At this time, theinductance value H1 becomes a relatively small value, and the switchingregulator 2 operates in the continuous mode. In the case of the lightload, the load determination circuit 30 outputs a high-leveldetermination signal 34, and the FET 17 turns off based on thedetermination signal S4. At this time, the inductance value H1 becomes arelatively large value. Therefore, the switching regulator 2 operates inthe continuous mode even when the load is light to some extent.

As described above, in the switching regulator 2 according to thepresent embodiment, the load determination circuit 30 includes theintegration circuit which is configured by the resistor 21 and thecapacitor 22 and outputs the average voltage of the voltage of thecontrol terminal of the switching element, and the buffer 31 whichoutputs the determination signal S4 based on the output signal of theintegration circuit. Therefore, the load determination circuit 30 whichoutputs the determination signal S4 in accordance with the load can beconfigured easily by using the integration circuit and the buffer 31.

In the switching regulator 2 according to the present embodiment, afirst conduction terminal of the switching element (source terminal ofthe FET 18) is connected to the input terminal, a first terminal of therectifier element (anode terminal of the diode 13) and another electrodeof the output capacitor (capacitor 14) are grounded, a second conductionterminal of the switching element (drain terminal of the FET 18), asecond terminal of the rectifier element (cathode terminal of the diode13), and a first terminal of the variable inductance circuit 10 areconnected to a first node (N2 shown in FIG. 4), and a second terminal ofthe variable inductance circuit 10 is connected to the output terminal.Therefore, according to the switching regulator 2 according to thepresent embodiment, as with the first embodiment, a step-down switchingregulator which operates efficiently even when the load is light to someextent can be provided without greatly increasing the circuit size andthe cost.

Third Embodiment

FIG. 5 is a circuit diagram of a switching regulator according to athird embodiment. A switching regulator 3 shown in FIG. 5 includes avariable inductance circuit 40 (including the two coils 11, 16), the FET18, the diode 13, the capacitor 14, the control circuit 19, and the loaddetermination circuit 20. The switching regulator 3 is obtained based ona general inverting switching regulator using the chopper method byreplacing the coil 11 with the variable inductance circuit 40 and addingthe load determination circuit 20.

As shown in FIG. 5, the variable inductance circuit 40 is obtained byreplacing the FET 17 in the variable inductance circuit 10 with an FET41. The FET 41 is an N-channel type FET and functions as a secondswitching element. In the following, an inductance value of the variableinductance circuit 40 is denoted by H2.

The source terminal (left-side terminal) of the FET 18 is connected toan input terminal of the switching regulator 3. The drain terminal ofthe FET 18 is connected to a first terminal (upper-side terminal) of thevariable inductance circuit 40 and the cathode terminal of the diode 13.The anode terminal of the diode 13 is connected to one electrode(upper-side electrode) of the capacitor 14 and an output terminal of theswitching regulator 3. A second terminal of the variable inductancecircuit 40 and another electrode of the capacitor 14 are grounded.

The input terminal of the control circuit 19 is connected to the outputterminal of the switching regulator 3, and the output terminal of thecontrol circuit 19 is connected to the gate terminal of the FET 18. Theoperation of the control circuit 19 is same as that of the secondembodiment. The configuration of the load determination circuit 20 issame as that of the first embodiment. The output terminal of theinverter 23 is connected to a gate terminal of the FET 41.

In the case of the heavy load, in order to increase the current flowingthrough the coil 11, the control circuit 19 controls the low-levelperiod of the gate voltage of the FET 18 to be long. Thus, the outputvoltage of the integration circuit configured by the resistor 21 and thecapacitor 22 comes close to the low-level voltage VL, and an outputsignal S5 of the inverter 23 becomes high-level. Since the FET 41 turnson at this time, the current flowing through the coil 11 flows throughthe FET 41, but does not flow through the coil 16. Therefore, theinductance value H2 in the case of the heavy load is equal to theinductance value of the coil 11.

In the case of the light load, in order to decrease the current flowingthrough the coil 11, the control circuit 19 controls the low-levelperiod of the gate voltage of the FET 18 to be short. Thus, the outputvoltage of the integration circuit comes close to the high-level voltageVH, and the output signal S5 of the inverter 23 becomes low-level. Sincethe FET 41 turns off at this time, the current flowing through the coil11 flows through the coil 16, but does not flow through the FET 41.Therefore, the inductance value H2 in the case of the light load isequal to the sum of the inductance value of the coil 11 and theinductance value of the coil 16.

In this manner, in the case of the heavy load, the load determinationcircuit 20 outputs a high-level determination signal S5, and the FET 41turns on based on the determination signal S5. At this time, theinductance value H2 becomes a relatively small value, and the switchingregulator 3 operates in the continuous mode. In the case of the lightload, the load determination circuit 20 outputs a low-leveldetermination signal S5, and the FET 41 turns off based on thedetermination signal S5. At this time, the inductance value H2 becomes arelatively large value. Therefore, the switching regulator 3 operates inthe continuous mode even when the load is light to some extent.

As described above, in the switching regulator 3 according to thepresent embodiment, a first conduction terminal of the switching element(source terminal of the FET 18) is connected to the input terminal, afirst terminal of the rectifier element (anode terminal of the diode 13)is connected to the output terminal, a second conduction terminal of theswitching element (drain terminal of the FET 18), a second terminal ofthe rectifier element (cathode terminal of the diode 13), and a firstterminal of the variable inductance circuit 40 are connected to a firstnode (N3 shown in FIG. 5), and a second terminal of the variableinductance circuit 40 and another electrode of the output capacitor(capacitor 14) are grounded. Therefore, according to the switchingregulator 3 according to the present embodiment, as with the first andsecond embodiments, an inverting switching regulator which operatesefficiently even when the load is light to some extent can be providedwithout greatly increasing the circuit size and the cost.

Fourth Embodiment

FIG. 6 is a circuit diagram of a switching regulator according to afourth embodiment. A switching regulator 4 shown in FIG. 6 is obtainedby replacing the load determination circuit 20 in the switchingregulator 1 according to the first embodiment with a load determinationcircuit 50.

As shown in FIG. 6, the load determination circuit 50 includes resistors21, 51 to 53, the capacitor 22, and a comparison circuit 54. One end(upper-side terminal) of the resistor 51 is connected to the sourceterminal of the FET 12. One end (right-side terminal) of the resistor 21is connected to the gate terminal of the FET 12. Another end of theresistor 21 is connected to one electrode (upper-side electrode) of thecapacitor 22 and a minus-side input terminal of the comparison circuit54. A reference voltage Vref is supplied to one end (upper-sideterminal) of the resistor 52. Another end of the resistor 52 isconnected to one end (upper-side terminal) of the resistor 53 and aplus-side input terminal of the comparison circuit 54. Another electrodeof the capacitor 22 and another ends of the resistors 51, 53 aregrounded. An output terminal of the comparison circuit 54 is connectedto the gate terminal of the FET 17. The resistors 52, 53 function as aresistor divider circuit which generates a comparison target voltagebased on the reference voltage Vref.

In the case of the heavy load, in order to increase the current flowingthrough the coil 11, the control circuit 15 controls the high-levelperiod of the gate voltage of the FET 12 to be long. Thus, the outputvoltage of the integration circuit configured by the resistor 21 and thecapacitor 22 comes close to the high-level voltage VH, and an outputvoltage of the comparison circuit 54 becomes low. Since the FET 17 turnson at this time, the current flowing through the coil 11 flows throughthe FET 17, but does not flow through the coil 16. Therefore, theinductance value H1 in the case of the heavy load is equal to theinductance value of the coil 11.

In the case of the light load, in order to decrease the current flowingthrough the coil 11, the control circuit 15 controls the high-levelperiod of the gate voltage of the FET 12 to be short. Thus, the outputvoltage of the integration circuit comes close to the low-level voltageVL, and the output voltage of the comparison circuit 54 becomes high.Since the FET 17 turns off at this time, the current flowing through thecoil 11 flows through the coil 16, but does not flow through the FET 17.Therefore, the inductance value H1 in the case of the light load isequal to the sum of the inductance value of the coil 11 and theinductance value of the coil 16.

In this manner, in the case of the heavy load, the load determinationcircuit 50 outputs the determination signal S6 having a low voltagelevel, and the FET 17 turns on based on the determination signal S6. Atthis time, the inductance value H1 becomes a relatively small value, andthe switching regulator 4 operates in the continuous mode. In the caseof the light load, the load determination circuit 50 outputs thedetermination signal S6 having a high voltage level, and the FET 17turns off based on the determination signal S6. At this time, theinductance value H1 becomes a relatively large value. Therefore, theswitching regulator 4 operates in the continuous mode even when the loadis light to some extent.

As described above, in the switching regulator 4 according to thepresent embodiment, the load determination circuit 50 includes theintegration circuit which is configured by the resistor 21 and thecapacitor 22 and outputs the average voltage of the voltage of thecontrol terminal of the switching element, a resistor divider circuit(resistors 52, 53) which generates a comparison target voltage based onthe reference voltage Vref, and the comparison circuit 54 which comparesthe output voltage of the integration circuit with the comparison targetvoltage to output the determination signal S6. Therefore, the loaddetermination circuit 50 which outputs the determination signal S6 inaccordance with the load can be configured easily by using theintegration circuit, the resistor divider circuit, and the comparisoncircuit 54.

Fifth Embodiment

FIG. 7 is a circuit diagram of a switching regulator according to afifth embodiment. A switching regulator 5 shown in FIG. 7 is obtained byreplacing the variable inductance circuit 10 in the switching regulator1 according to the first embodiment with a variable inductance circuit60. In the following, an inductance value of the variable inductancecircuit 60 is denoted by H3.

As shown in FIG. 7, the variable inductance circuit 60 includes coils61, 62 and the FET 17. The coil 61 functions as a first coil, and thecoil 62 functions as a second coil. One end (left-side terminal) of thecoil 61 and the source terminal of the FET 17 are connected to an inputterminal of the switching regulator 5. The drain terminal of the FET 17is connected to one end (left-side terminal) of the coil 62. Another endof the coil 61 and another end of the coil 62 are connected to the drainterminal of the FET 12 and the anode terminal of the diode 13.

In this manner, the FET 17 is connected in series with the coil 62. Thecoil 61 is connected in parallel with a series connection circuit of thecoil 62 and the FET 17. A first terminal (left-side terminal) of thevariable inductance circuit 60 is connected to the input terminal of theswitching regulator 5. A second terminal of the variable inductancecircuit 60 is connected to the drain terminal of the FET 12 and theanode terminal of the diode 13. In the following, as an example, it isassumed that an inductance value of the coil 61 is 100 μH, and aninductance value of the coil 62 is 10 μH.

As with the first embodiment, in the case of the heavy load, the outputsignal S2 of the inverter 23 becomes low-level. Since the FET 17 turnson at this time, a current flows through both of the coils 61, 62.Therefore, the inductance value H3 in the case of the heavy load isequal to a composite inductance value of the coils 61, 62 connected inparallel (9 μH). In the case of the light load, the output signal S2 ofthe inverter 23 becomes high-level. Since the FET 17 turns off at thistime, the current flows only through the coil 61. Therefore, theinductance value H3 in the case of the light load is equal to theinductance value of the coil 61 (100 μH).

In this manner, in the case of the heavy load, the load determinationcircuit 20 outputs the low-level determination signal S2, and the FET 17turns on based on the determination signal S2. At this time, theinductance value H3 becomes a relatively small value, and the switchingregulator 5 operates in the continuous mode. In the case of the lightload, the load determination circuit 20 outputs the high-leveldetermination signal S2, and the FET 17 turns off based on thedetermination signal S2. At this time, the inductance value H3 becomes arelatively large value. Therefore, the switching regulator 5 operates inthe continuous mode even when the load is light to some extent.

As described above, in the switching regulator 5 according to thepresent embodiment, the variable inductance circuit 60 includes a firstcoil (coil 61), a second coil (coil 62), a second switching element (FET17) which is connected in parallel with the second coil, turns on whenthe determination signal S2 indicates the heavy load, and turns off whenthe determination signal S2 indicates the light load, and the first coilis connected in parallel with a series connection circuit of the secondcoil and the second switching element. Therefore, the variableinductance circuit 60 having an inductance value which is switchablebased on the determination signal S2 can be configured easily.

As for the above-described switching regulators, various kinds ofmodifications can be configured. For example, switching regulatorsaccording to modifications of the second to fourth embodiments mayinclude the same variable inductance circuit as that of the fifthembodiment. Furthermore, in a variable inductance circuit of a switchingregulator according to a modification, a first coil, and a parallelconnection circuit of a second coil and a second switching element maybe connected in a reverse order, or the second coil and the secondswitching element may be connected in a reverse order. Furthermore, aswitching regulator according to a modification may include an FET whichmakes a current flow in a same direction as the diode 13 when it is ON,as the rectifier element in place of the diode 13. Furthermore, in aswitching regulator according to a modification, a load determinationcircuit may include any of an inverter, a buffer, and a comparisoncircuit in accordance with a configuration of a variable inductancecircuit. Furthermore, in a switching regulator according to amodification, a variable inductance circuit may include a plurality ofcoils connected in series or in parallel, in place of the first andsecond coils. Furthermore, in a switching regulator according to amodification, a switching regulator IC may include a control circuit anda load determination circuit.

Although the present invention is described in detail in the above, theabove description is exemplary in all of the aspects and is notrestrictive. It is understood that various other changes andmodification can be derived without going out of the present invention.

What is claimed is:
 1. A switching regulator comprising: a switchingelement; a rectifier element; an output capacitor having one electrodeconnected to an output terminal; a control circuit configured to supplya pulse width modulation signal in accordance with a voltage of theoutput terminal to a control terminal of the switching element; a loaddetermination circuit configured to output a determination signal inaccordance with a load, based on a voltage of the control terminal ofthe switching element; and a variable inductance circuit including aplurality of coils and having an inductance value which is switchablebased on the determination signal, wherein the load determinationcircuit includes an integration circuit configured by a resistor and acapacitor and configured to output an average voltage of the voltage ofthe control terminal of the switching element.
 2. The switchingregulator according to claim 1, wherein the variable inductance circuitincludes: a first coil; a second coil; and a second switching elementconnected in parallel with the second coil and configured to turn onwhen the determination signal indicates a heavy load and to turn offwhen the determination signal indicates a light load, and the first coilis connected in series with a parallel connection circuit of the secondcoil and the second switching element.
 3. The switching regulatoraccording to claim 1, wherein the variable inductance circuit includes:a first coil; a second coil; and a second switching element connected inseries with the second coil and configured to turn on when thedetermination signal indicates a heavy load and to turn off when thedetermination signal indicates a light load, and the first coil isconnected in parallel with a series connection circuit of the secondcoil and the second switching element.
 4. The switching regulatoraccording to claim 1, wherein the load determination circuit furtherincludes an inverter configured to output the determination signal basedon an output signal of the integration circuit.
 5. The switchingregulator according to claim 1, wherein the load determination circuitfurther includes a buffer configured to output the determination signalbased on an output signal of the integration circuit.
 6. The switchingregulator according to claim 1, wherein the load determination circuitfurther includes: a resistor divider circuit configured to generate acomparison target voltage based on a reference voltage; and a comparisoncircuit configured to compare an output voltage of the integrationcircuit with the comparison target voltage to output the determinationsignal.
 7. The switching regulator according to claim 1, wherein a firstterminal of the variable inductance circuit is connected to an inputterminal, a second terminal of the variable inductance circuit, a firstconduction terminal of the switching element, and a first terminal ofthe rectifier element are connected to a first node, a second terminalof the rectifier element is connected to the output terminal, and asecond conduction terminal of the switching element and anotherelectrode of the output capacitor are grounded.
 8. The switchingregulator according to claim 1, wherein a first conduction terminal ofthe switching element is connected to an input terminal, a firstterminal of the rectifier element and another electrode of the outputcapacitor are grounded, a second conduction terminal of the switchingelement, a second terminal of the rectifier element, and a firstterminal of the variable inductance circuit are connected to a firstnode, and a second terminal of the variable inductance circuit isconnected to the output terminal.
 9. The switching regulator accordingto claim 1, wherein a first conduction terminal of the switching elementis connected to an input terminal, a first terminal of the rectifierelement is connected to the output terminal, a second conductionterminal of the switching element, a second terminal of the rectifierelement, and a first terminal of the variable inductance circuit areconnected to a first node, and a second terminal of the variableinductance circuit and another electrode of the output capacitor aregrounded.
 10. The switching regulator according to claim 1, wherein therectifier element is a diode.